Fluidic liquid level detector



United States Patent 2 lnvemor Ed i us 1" 3,456,668 7/1969 Wheeler137/815 Silver Spring, Maryland 3,458,129 7/1969 Woodson l37/81.5X [21]Appl. No. 782,326 3,467,122 9/1969 Jones l37/8l.5 [22] Filed Dec. 9,19683,468,325 9/1969 Bowles l37/8l.5 [45] Patented Nov. 24, 1970 P E [73]Assignee Bowles Engineering Corporation :;Z ;:'x; gs z f ggg SilverSpring, Maryland a corporation of Maryland [54] FLUIDIC LIQUD LEVELDETECTOR ABSTRACT:- A liquid level detector in a preferred embodiwchim's 3 Drawing ment, comprises a fluidic oscillator having a singlefeedback line which when blocked by liquid induces oscillations in the[52] US. Cl. 137/815 oscillator and h open to air inhibits osci||ationsThe osci| l 1/10 lator output signal is divided, one-path feeding arectifier and [50] Field olSearch [37/8 l .5; filter and a second pathfeeding a resonant circuit and a recti- P- w -rpfier and filter. Theresonant circuit is tuned to the nominal operating frequency of theoscillator so that when the oscilla- [56] References cued tor isoscillating the two output paths provide signals at dif- UNlTED STATESPATENTS ferent pressure levels and when the oscillator is inhibited the3,204,652 9/1965 Bauer..' l37/8i.5 two output paths providesubstantially equal pressure levels. 3,340,885 9/ 1967' 137/815 Theoscillator frequency is variable with the height of liquid in 3,379,2044/1968 l37/81.5 the feedback line so that the pressure differentialprovided 3,398,758 8/1968 l37/8l.5 between the two output paths varieswith the level of the 3,448,752 6/1969 l37/8l.5 liquid.

3| 33 R L C RECTAE! ER 37 CIRCUIT FILTER l sunnme T-'- applications No.

. 1 rLurmcu uln LEVEL DETECTOR BACKGROUND OF THE INVENTION The presentinvention relates to fluidic liquidlevel detection, and moreparticularly to afluidic circuit-in'which oscillatory fluid signals areprovided and liquid level. v Prior art fluidic liquid leveldet'ectorsare of the .fgo-no go" type, providing a first fluid" pressure when thesensed liquid level blocks themouthof a sensor tube and a second fluidpressure when the rn'o'uthof the sensor tube is terminated in sensed asa function of a 2 BRIEF DESCRIPTION OF THE DRAWINGS The above and stillfurther objects, features and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionof the specific embodiments thereoflespecially when taken in conjunctionwith the accompanying drawings, wherein:

' FIG. 1 is a'schematic drawing of a liquid level detector circuitconstructed in accordance with the principles of the air. Examples ofsuch devices maybe found in U.S. Pat. No. 3,267,949 to Adams, U.S. Pat.No; 3,277,194 to Manion; U.S.

Pat. application Ser. No. 490,246, filed Sept. 27, I965,- and U.S. Pat.application Ser. -No. 7l3,480 filed fMar. 15, 1968.

None of the above-described prior art devices are capable I of providinga signal pressure which varies in proportionto-the level of the liquidbeing-detected. It is evident a signal which so varies would be useful,both for measuring the level of the liquid and for initiating controlfunctions as required upon the liquidachieving predetermined levels. Forexample, in the above-referenced U.S. Pat. applications No.49 0,246 andNo. a

7 l3,480 systems are disclosed which function to maintain the level of aliquid between two predetermined levels, attainment of said levels beingsensed by two respectivefluidic sensors of the go-no go" type. Asubstantial reduction in cost and space would be achieved by a singleliquid level detector which provides a signal pressure which variesinproportion to the level of the liquid and therefore could triggerappropriate control operations as the liquid attains the predeterminedlevels.

It is therefore an objectof the present invention to provide a fluidicliquid level detector having an output signal which-varies as a knownfunction of the level of the'liquid being detectcd. i i

it is a further object of thepresent invention to provide a fluidicliquid level detector having two alternative operational i modes:onemode in which the detector output signal is a present invention;

FIG. 2 is a schematic drawing of a modified version of the liquid leveldetector of FIG. I; and

FIG. 3 is a schematic drawing of a system employing the liquid leveldetector of the present invention to maintain the liquid in a tankbetween two predetermined levels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring nowspecifically to FIG. 1 of the accompanying drawings, there isillustrated a tank 10 containing a liquid, the level of which is to bemonitored. A fluidic oscillator element 11 comprises an interactionregion 13, a power nozzle 15, a

control nozzlel7 and a pair of. output passages 19, 21. The powernozzle'15 is responsive to application of pressurized fluid thereto toissue a power stream into interaction region 13 generally towardoutputpassages 19,21. Control nozzle 17 is responsive to application ofpressurized fluid thereto to issue a 7 'below level 3-8 (for example atlevel A-A), the end of senknown function of thegldetectedliqu'idleveland-asecond mode in which the detectormerelyj providesan'iindication as l to whether or not the liquid isator'abovea'specified level.

SUMMARY OF THE INVENTION In accordance with the principles of thepresent invention, liquid leveldetection is accomplished 'by rnonitoringoscillations in a fluidic oscillator. The oscillator itself may takemany configurations, but, mostimportantly, is nonoscillatory when theliquid isbelow a predetermined level and oscillatory when the liquid isabove the predetermined leveL The oscillator prosor tube 23 isterminated by the relatively low impedance air in tank 10. The powerstream, in such a case, aspirates air from tank 10 via sensor tube23,feedback passage 25 and control nozzle'l7. No such aspiration pathisprovided on the side of the power stream opposite control nozzle 17 sothat a pressure differential is developed across the power stream,deflect ing the latter toward output passage 21. Because of the boundarylayer phenomenon, the power stream locks onto the sidewall of chamber 13which in part defines passage 21, and

9 as long'aspassage 19 remains terminated by air.

vides an output signal which divides between two;sign al paths.

One pathcomprise's a'resonant circuit in serieswith a rectifier andsmoothing filter. .The second path simply comprises arectifier andsmoothing filter. When the oscillator is nonoscillatory, a steady fluidsignal is applied to each path, the steady signal beingsubstantiallyunaffected by the resonant circuit and equally affected bythe two rectifier-filtercombinations to provide substantiallyzeropressure differential across the out:

put ports or the two pathsT-whenl the oscillator is oscillatory, I

the resonant circuitamplifies (or attenuates if desired)'the oscillatorysignal, thereby providin'g asignificant-pressure differential across the'outputports ,of the two paths.

If the oscillator is of the-type in which the level .of the liquid inthe sensor tube determines the operating frequency of the oscillator,the variable 'frequencysignal experiences variable amplification (or'attenuation) in the resonant circuit to produce a variable pressuredifferential'across the two signal paths. If the frequency variation ofthe'oscillator in response to liquid level changesistoo small to bedetected by the resonant circuit, the oscillatorysignal maybeheterodyned against a reference oscillator signal. and the differencefrequency variation may so be detected by a resonant circuit.

the power stream remains stably directed toward said passage If thelevel of the liquid intank 10 is raised to level 8-8 so that sensor tube23 is terminated by the relatively high impedance liquid, aspiration ofair from tank 10 into interaction chamber 13 ceases. Nevertheless, thepower stream continues to entrain the air remaining in sensor tube'23,feedback passage 25 and output passage 19 until the pressure to the leftof the power stream (as. viewed in FIG. 1) becomes sufficiently low tocause.thepower stream to switch over to passage 19. Upon switching ofthe powerstrea'm, a sonic pres- I sure pulse travels the combinedlengths of passages 19 and 23 until it reaches thelevel 3-8 of theliquid. The sonic pressure pulse is reflected by the liquid level backthrough sensor tube 23, into feedback passage 25 and throughcontrolnozzle 17 to redeflect the power stream back to output passage21. Once again the power stream begins aspirating the fluid remaining inpassages 19, 23 and 25 until the pressure at the ingress of passage 19is sufficiently low to cause the power stream to once again switch topassage 19. It is evident that an oscillation mode is developed due tothe closure of sensor tube 23 by.

' ,the liquid at level BB'-, and that when the liquid is below levelB-B" a steady or d. c. signal is provided at output passage 21.

Output passage 21 divides into two separate flow paths, 27 and 29 suchthat substantially equal flow division of the fluid in passage 21 isachieved. Signals traversing path 27 are fed to a resonant circuit' 31and a rectifier-filter circuit 33, connected in series. Resonant circuit31 comprises R, L, and C (resistance, inertance and capacitancerespectively) components interconnected to provide an impedancecharacteristic which varies in a known manner with the frequency ofsignals passing through the circuit. Circuits of this type andtechniques for selecting desired impedance versus frequencycharacteristics are disclosed in U.S. Pat. No. 3,292,648. For purposesof the present discussion, it is assumed that circuit 31 is resonant atthe frequency at which oscillator 11 operates when the liquid in tank isat level 8-5. It will be understood, of course, that this choice ofimpedance versus frequency characteristic is by no means limiting on thescope ofthe present invention. The Q factor of the resonant circuit isassumed to be such-that a signal at the resonant frequency is amplifiedsubstantially by circuit 31 relative to a nonalternating or d. c.signal. In fact, d. c. signals are assumed to pass through resonantcircuit 31 substantially unaffected.

Rectifier and filter circuit 33 may be of the type disclosed in U.S.Pat. No. 3,292,648 and function to convert oscillatory or a. c. signalsto d. c. signals of corresponding amplitude. A similar rectifier andfilter circuit 35 is provided in flow path 29, and the two flow pathsterminate in respective opposing control ports of a summing amplifier37. The latter, by way of example, may be an analog fluidic amplifier ofthe stream interaction type which provides a differential pressureoutput signal A P, as a function of the differential pressure betweenrespective output ports of rectifier and filter circuits 33 and 35.

In operation, the circuit of FIG. 1 provides a zero pressuredifferential across the output ports of amplifier 37 when the liquid intank 10 is below level 8-8. This is because oscillator 11 isnonoscillatory and provides a d. c. signal at passage 21 which isequally divided between paths 27 and 29. Resonant circuit 31 hasnegligible effect on d. c. signals and any losses introduced byrectifier-filter circuits 33, and 35 affect the divided signals equally.Consequently, equal signals appear at the control ports of amplifier 37and A P, is zero.

When the liquid intank 10 is at level B-B', oscillator 11 assumes itsoscillatory mode and an a. c. signal at the resonant frequency ofcircuit31 is applied to bothflow paths 27 and 29. The resonant circuit 31amplifies the signal applied to path 27 but no such amplification iseffected in path 29. Thus, the rectified and filtered d. c. signalprovided by rectifier-filter circuit 33 is of significantly greateramplitude than the d. c. signal provided by rectifier filter circuit 35.The A P, output signal from amplifier 37 thus assumes some appreciablelevel which can be employed for indication and/or control purposes.

As thus far described, the circuit of FIG. I is seen to perform a go-nogo detection function, providing a monitorable signal, A P,,, toindicate whether the liquid in tank 10 is at or below level 8-8. Uponexamination of oscillator 11 however it is noted that the frequency ofthe oscillator signal is variable as a function of the liquid level intank 10 whenever the liquid is at or above level B-B'. Specifically, thefrequency of operation of oscillator 11 is determined by the timerequired for a pressure pulse to travel down through output passage 19and sensor tube 23 and then be reflected back up through sensor tube 23and feedback passage to control nozzle 17. This time is varied as thelevel of the liquid in sensor tube 23 varies; this is because thepressure pulse reflects off the liquid surface and the point ofreflection thus varies with the height of the liquid in tube 23. Thus,when the liquid in tank I0 is at level C-C', above level 8-8, thetransit time for the pressure pulse is substantially shorter than whenthe liquid is at level 8-8. Hence, the frequency of oscillator 11 issubstantially higher when the liquid is at level C-C' than when it is atlevel 8-8. In fact, the frequency of oscillator 11 varies linearly withvariations of the liquid level in sensor tube 23. If resonant circuit 31is selected to provide a known impedance versus frequency characteristicover the operating frequency range of oscillator 11, the amplitude ofthe signal in flow path 27 will vary with frequency in accordance withthat characteristic while the amplitude of the signal in path 29 remainsunaffected by frequency variations. The output signal A P, fromamplifier 37 therefore provides a continuous measure of level of liquidin sensor tube 23.

It is to be noted that the specific configuration illustrated in FIG. 1for oscillator 11 is by no means a limiting factor on the scope of thepresent invention. Any fluidic oscillator may be employed within theprinciples of the present invention so long as it provides a d. c.output signal when the liquid in tank 10 is below the open end of sensortube 23 and provides an oscillatory output signal when the liquid intank 10 blocks the end of sensor tube 23. Examples of such oscillatorsmay be found in U.S. Pat. No. 3,204,652, and in copending U.S. Pat.application Ser. No. 705,345 by Peter Bauer, filed Feb. 14, 1968,entitled Condition Responsive Pure Fluid Oscillator and assigned to thesame assignee as the present invention. Oscillators need not provide avariable frequency versus liquid level characteristic to be within thescope of the present invention, since for some applications a simplego-no go device may be sufficient.

It is also to be noted that circuit 31 need notbe tuned to resonate atthe frequency of oscillator 11 which corresponds to level B-B'. Any RLCcircuit which provides a variable impedance versus frequencycharacteristic within the operating frequency range of oscillator 11 andwhich has negligible effect on d. c. signals may be employed.

It is to be further noted that path 29 may be eliminated if the systemin which the present invention is employed requires an output signal ona single line rather than a differential pressure signal. Thisconfiguration is illustrated in FIG. 2 which additionally illustrates afurther modification of the circuit of FIG. 1. More specifically, it ispossible that for some applications of the level detector of the presentinvention, the variation in liquid level to be monitored is so small asto render the concomitant frequency variation too small to be readilydetected by circuit 31. Under such circumstances, the output signal maybe heterodyned or beat" with a reference frequency signal to provide adifference frequency which varies with the frequency of oscillator 11but over a proportionately greater range.

Referring specifically to FIG. 2, tank 10 is illustrated with sensortube 23 extending thereinto from oscillator 11. Oscillator 11 functionsas described above in reference to FIG. 1 to provide an a. c. signalhaving a frequency f which varies with the liquid level in tank 10. Areference oscillator 41 provides a fluid a. 0. signal at a constantfrequency f,, and both signals (f and f,) are applied to a frequencymixer 43. Mixer 43, by way of example, may be of the type disclosed inU.S. Pat. No. 3,292,648, and functions to provide an a. c. signal at afrequency (f f,) equal to the difference between the two inputfrequencies. The mixer output signal is then applied to a single flowpath comprising RLC circuit 31 and rectifier filter circuit 33,corresponding to flow path 27 in FIG. 1. If a differential pressureoutput signal is desired, the output signal from mixer 43 can be dividedbetween two paths as is the output signal from oscillator 11 in FIG. 1.

To illustrate the advantage accruing by virtue of the heterodyningfeature employed in the circuit of FIG. 2, assume that fis variable overthe range from I05 to I 10 Hz. The maximum variation is somewhat lessthan 5 percent and very well'may not be great enough to provide asubstantial output variation from circuit 31 in FIG. 1. If the referencefrequency f, is chosen as Hz, the output frequency f f, from mixer 43varies over the 5 to 10 Hz range, a variation of I00 percent whichshould be readily detectable.

Another point to note concerning the operation of the embodimentillustrated in FIG. 2 is that when the liquid in tank 10 is below thelevel of sensor tube 23, the mixer output signal is not a d. c. signalbut rather an a. c. signal at frequency f,. However, f, is sufficientlyremoved from the operating range of f f, to produce a substantiallydifferent output signal amplitude from circuit 31 than is the case whenthe liquid in tank 10 is at or above the level of the open end of sensortube 23.

' state. Consequently,

' circuit 31 and rectifier-filtercircuit 33 'are' provided and-functionto providead. c. output'signal from rectifier-filter circuit 33 havinganarnplitude whichvaries with thelevei of liquid in tank 10. This d; c.outputfsignal. is applied; to a pair of threshold gates 51 and 53',' forexample ofthe fluidic OR/NOR type disclosedin U.- S P at: No. 3,340,885.Gates Slfand 53' each assume one binaryfstate' when their input 'signal'arn plitudeis below apr'edetermined threshold-anda second binary statewhen' the input signal amplitude is abovethat threshold. The thresholdlevel is adjustable-and it is assumed that the threshold for gate 51corresponds tothe signal level at the output portof'rect'ifier-flltercircuit 33 when theliquid in tank is at level' D ljfy-it is furtherassunied that the threshold for gate 5 3fcorrespon ds to the signallevel at the out to place thelatterin one of its two stable states. Whengate 53 is in its OR mode (thatfis, when'theinpu't signalto gate53'isequal to or greater than thelthreshold'of gate 53) a signal is ap--plied to flip flopj'5 5ito place the latter inthe second of its'twostable states. During the first mentioned stable state 'of-flipu .30responds by pumping liquid into tank 10. When flip-flop 55 is signal isapflop 55 an actuating signal is appliedlto-a pump 57which in the secondof its two stable states, no actuating plied to pump 57 and liquidfllling'oftankl fllceases.

Assume that t'h'e frequency characteristic of circuit 31 is such that asignal ofin'creasing amplitude is provided thereby V I j r I Ifrequency-of oscillationjof saidfluidic oscillator means varies Whengate 51' isin itsNO R modei'that is, when the signal applied to gate 5.1is below the-thresholdof gate '5 1 )a signal is applied to one'side ofa'fluidic bistableelernen'torflip-flop'55' ing further fluid circuitmeans responsive to the fluid output signal from said fluidic oscillatormeans for providing a fluid signal atsaid first nonalternating amplitudewhen said output signal is at a substantially constant amplitude and ata third nonalternating amplitude when said output signal is at analternati'ng amplitude.

3. The fluidic sensor accordingto claim Zutilized as a liquid levelsensor wherein closure of said sensingen'd of said tube is effected byattainment of a predet'e'rmiriedlevel by. liquid-in a tank, said fluidicsensor'being further'characterized in that the fluid signal provided bysaid further fluid circuit means has an amplitude'which'does notvarywith the frequency of the out in responsetoincreasingliquid.level-intankdtl. llllhen theliquid is belowlevelDf-D'i Signal applied to :gatesfa'l'and l 53 is below thethresholdofboth' gates,.,and gate 51 applies a signal to.flip-flopfSSito 'place theqlatter' in tits-pump-actuating fliquidisfadded to tank 10. when the liquid reaches level D.- D ';-'gate 1 51switchesxto its OR mode, removing the input sig'n'al .to flip-flopiSSliThe. latter. being bistable, however, remains-in itspump-actuating stateandthe addition of liquid to tank 10 continues. 5

When the liquid attains levelEi-E' gate 53 switches to its I OR mode andactuate's flip-flop to deactuate the pump and terminatethcfillin'g of'tank IOQ-The pump eannotnowbe reactuated until the liquid falls belowlevel D- Df at which time gate Si is switched to its NOR-mode and inturn switches flipflop 55 to its'pumpgac'tuating inode. It is apparent.thatth f system of FIG"; 3-ac'tsEto maintain thefliquid irr table-.10betweenlevels D-D'tlndE-E". t

As discussedabo ve; the fluidic level detector of the presentinventionmay operate in-either a go-F ogofif mddeorfih a mode which thelevelv ofthe liquidiscontinuously meatector mayb'e employed for functionsother than liquid level detecting. For example, oscillatorniay-be'employed to de{ tect closure of tube 23' by a finger, a bimetalstrip for temperature sensing. a'h'umidity senso'r, the armature .,ofa-relayor so1enoid,orasensedobject upon-proximitycetection., i

vided by'saidfl uid circuit means hasa nonaiternating amplitude whichvaries asa function of the frequency of the fluid output 'signalfromsaid fluidic oscillator rn'eans, and'in that the put'signal from saidfluidic oscillator means.

4. The fluidic sensor according to claim 1 utilized as a liquidleve'lsensor wherein closure of the sensing end of said tube is causedby the attainment of a predetermined level by liquid in a tank, saidfluidic sensor being further characterized in that the frequency ofoscillation of said fluidic oscillator means varieswith the level ofliquidin said tube, and in that'the fluid .Signal 'provided bysaid-fluid circuit means has a nonalternating amplitude which'varies asa function of the frequency of the fluid output signal from saidfluidicoscillator means.

5.; The fluidic-sensor according to claim 4 further comprising referenceoscillatormeans'for providing a fluid signal of alternating amplitudeat,afixed frequency, frequency mixer .jmearis for providing a fluid signalof alternating amplitudeat a frequency equal to-the "difference infrequency between the fluid'sign'al provided by'said reference o'scillatorjmeans and the fluid output signal provided 'by said fluidicoscillator means, and means-forapplyingthe fluid signal provided by saidmixer means to said fluid circuit means.

- 'sured. As to the former mode, it should be notedthat-the dc While.l-ha'vedescribedmnd illustrated thei specifieiembodi- I ments of myinvention, it willbe clear that variations-of the details ofconstruction 'which'are specifically illustrated and described may beresorted to, withoutdepartingfromthe true spirit and scope'of theinvention asdefined'in the appended claims.

lclaim:

1. A fluidic sensorcomprisings a tube having a-sensing endp u fluidicoscillator means fdr providing a fluid output signal at a substantiallyiconstant amplitude in response. to the sensing end ofsaid tube beingopento a gaseous environ ment and responsive to closure of said sensing endof said bistable fluidic means'responsive to said first fluid switchingsignal 'for assuming a first stable state and responsive to said secondfluid switching signal for assuming a second stable state; and

means-responsive to bistable fluidicmeans when in said first stablestate to addliquid to said tank.

7. .A fluidic sensor for monitoringthe level .of liquid in a liquidcontainer, comprising: a fluidicelement having a power nozzle responsiveto application of fluid thereto for issuing a power stream of fluid,first and second output passages disposed for receiving said powerstream, said first output passage comprising a sensor tube having itsdownstream and disposed in said container, first means for stablydeflecting said power stream toward said second'output passage when saiddownstream 'end of said sensor tube is not'blocked by liquid in saidtank, and second means responsive to blockage of said sensor tube bysaid liquid for oscillating said power stream between said first andsecond output passages.

8. The fluidic sensor according to claim 7 further comprising resonantfluid circuit means responsive to said power stream when stablydeflected toward said second output passage for providing a fluid signalat a first amplitude and responsive to saidpower stream when oscillatingto provide a fluid signal at a second amplitude.

9. The fluidic sensor according to claim 8 wherein said second meanscomprises means responsive to aspiration of fluid from said first outputpassage by said power stream when said power stream is directed towardsaid second output passage and when said sensor tube is blocked byliquid for deflecting said power stream to said first output passage,and means for directing pressure pulses received by said first outputpassage upon switching of said power stream thereto against the surfaceof liquid blocking said sensor tube and reflecting said pressure pulsesback for deflecting said power stream to said second output passage.

10. The fluidic sensor according to claim 9 wherein the time requiredfor said pressure pulses to deflect said power stream towards saidsecond output passage after said power stream is switched to said firstoutput passage is variable with the level of liquid in said sensor,whereby the frequency of oscillation of said power-stream is variablewith the level of liquid in said sensor tube, and wherein said fluidcircuit means is responsive to said power stream when oscillating toprovide a nonoscillatory fluid signal having an amplitude which varieswith the frequency of oscillation of said power stream.

